Activated carbon in a molded form (hereinafter referred to merely as an "activated carbon" or a "granular activated carbon") is widely used in various fields, for example, in pollution-control equipment for waste gas treatment, waste water treatment and so forth. It is also used in the food industry for the purification of oils and fats and alcohol drinks, and the chemical industry for gas purification and drug purification because of its superior adsorption characteristics resulting from its porous structure and because of its ease of activation.
When used in the above-described fields, activated carbon is usually charged to a column and used in the treatment of various gases or liquids. For this purpose, it is required for activated carbon to have such a high mechanical strength as to be capable of withstanding a continuous use-regeneration cycle and high adsorption characteristics. In producing such activated carbon using wood as a carbon source, the following method has heretofore been employed.
Wood is carbonized by heating and then pulverized. A binder such as coal tar and pitch is added to the carbonized wood in an amount of about 30% by weight. The resulting mixture is then molded, and calcined and activated to produce the final activated carbon product.
This conventional method, however, has various disadvantages as described below.
(1) It is necessary for the binder to be added in an amount of about 30% by weight since carbonized wood powder has great oil-adsorption properties. If, however, such a large amount of binder is added, the raw material becomes plastic at relatively low temperatures and thus high density pellets are difficult to produce by molding. This results in the production of a granular activated carbon having a poor mechanical strength.
(2) If the binder content of the raw material is large, the binder such as coal tar and pitch comes out on the surface during calcination. Thus, pellets formed fuse to each other or are combined together, forming larger masses. In order to prevent the fusion of pellets as described above, it is necessary to apply, for example, a surface oxidation treatment.
(3) The conventional method requires two carbonization steps: carbonization of the wood by heating and carbonization of the mold by calcination. This makes the process complicated, increases the amount of energy consumed, and finally raises the price of activated carbon.